Greenpoison app will now show on-screen instruction to put your device in DFU Mode

You will now be prompted to press and hold the sleep button for 3 secs [image below]

After 3 seconds continue holding the sleep button and press the Home button for the next 10 seconds .

Now release the sleep button after 10 seconds have passed. Keep holding the Home button for next 15 seconds before your device goes into DFU Mode and the jailbreak process will begin automatically . [try this two-three times to perfect the timing] . Allow the process to complete . It will now show some codes running on your device.

After the Greenpoison tool has completed the jailbreaking process , close the app and look for green Loader Icon on your device[image below].

Tap on the Loader Icon and install Cydia .

You have now jailbroken your iphone 4 on iOS 4.2.1 using Greenpoison . If you find any difficulty while following the above tutorial , Feel free to ask your question in the Jailbreak Forum . No Registration Required .

Update – No Cydia :

Many users are facing No Cydia after having successful jailbreak . For them use Redsnow to rejailbreak and only use the install cydia option. This will give an untethered jailbreak via Greenpoison and redsnow to fix cydia problem.